1. FIELD OF THE INVENTION
This invention relates to an in-line type color cathode ray tube wherein the deflection defocus of electron beans on the scanned screen caused by the non-uniformity of the deflecting fields are substantially corrected by using beam distortion correcting means.
2. DESCRIPTION OF THE PRIOR ART
An in-line type color cathode ray tube is provided with a shadow mask which is adjacent and opposite to a screen containing a plurality of different color phosphor elements formed on the face plate. An electron gun assembly is disposed in the neck portion of the tube for emitting three electon beams which lie in the same plane so as to pass through the mask aperture and impinge on the phosphor screen. These three electron beams in in-line arrangement are usually constructed so that the center beam passes along the tube axis and so that the pair of side beams are axially distant at equal distances from the tube axis. These three electron beams are converged and deflected by a convergence apparatus and a deflection yoke disposed about the outside of the cathode ray tube.
As compared with a conventional .DELTA. (delta)--gun type color cathode ray tube in which the three electron beams are emitted from the apices of an equilateral triangle, this kind of in-line gun type color cathode ray tube has advantages, for example, in that the convergence adjustment of the electron beams is rather simple with the result that the covergence apparatus and its circuit can be simplified. This is significant for a color cathode ray tube in which a phosphor screen is comprised of a plurality of different color phosphor stripes.
However, this in-line type color cathode ray tube has the disadvantage that deflection defocus readily occurs in the electron beams because of the special non-uniform magnetic field of the deflection yoke which is necessary in order to obtain the required properties of the rasters. Accordingly, the picture quality of this tube deteriorates readily, especially in the peripheral portions of the screen.
A major cause of the deflection defocus of the in-line type color picture tube can be explained as follows. It is known that the deflection defocus of the electron beams is usually proportional to the nth power of the axial separation distance Sg of the electron beams (n.gtoreq.1.3). As the axial separation distance Sg of the side beams in in-line arrangement is greater by .sqroot.3 times as compared with the three electron beams in delta arrangement, the deflection defocus of the side beam readily occurs. Furthermore, the nonuniformity of the deflection fields and the convergence fields plays a significant part in this deflection defocus.
FIG. 1 is a diagram illustrating the side beam distortion caused by the non-uniformity of the deflecting field. Because of the pin-cushion type horizontal deflection fields 4, the electron beam 1 is subjected to forces which are not uniform at each point. As the beam is deflected toward the peripheral portion of the screen, its shape is deformed approximately into an ellipse 2 of which the longitudinal axis is in the horizontal direction. Moreover, as regards the side beam 1 shown in the drawing, the extend of its deflection defocus is not symmetrical on the right and on the left of the screen. The curve 6 shows a magnetic flux density illustrating the non-uniformity of the pin-cushion shaped horizontal deflection field.
This deflection defocus appears to be also caused by the non-uniformity of the convergence field. In particular, the side beams passing through the dynamic convergence magnetic fields are influenced to a great extent. Accordingly, the deflection defocus due to the aforesaid deflection fields in further increased, and the shape of the side beams become longer and thinner at the peripheral portions of the screen as shown in FIG. 2. In most color picture tubes, the screen ratio is approximately 4 : 3, and therefore, the abovementioned deflection defocus of the side beams appears to a great extent in the horizontal direction and in the diagonal direction.
As described above, a color picture tube in which the electron beams are arranged in-line readily produces the deflection defocus caused by the deflection field and by the convergence field. Accordingly, if the electron beams are deflected in a horizontal plane, the beam spots 2 become elongated in the horizontal direction as shown in FIG. 2. Consequently, the picture resolution of this in-line type color picture tube is greatly impaired at the peripheral portion of the screen. Furthermore, as the beam spot diameter in the vertical direction becomes smaller, a moire pattern readily appears in this tube.
Another important fact is that the beam spot size differs from point to point. This disadvantage is especially apparent at high beam current. Although proper convergence appears at low beam current, misconvergence appears at relatively high beam current, and the three-color images appear to slip.
It is known to design the aperture shape of the electrode of an electron gun to be an ellipse to reduce the deflection defocus. But this expedient for improving the structure of the electron gun electrode adversely changes the focusing property of the electron gun and also the picture resolution property of the color picture tube. Futhermore, it is difficult to construct a gun assembly using an electrode with an ellipse shaped aperture as compared to using a circular shaped aperture electrode. Therefore, improvement is not easily achieved and satisfactory results are not necessarily obtained.